1. Field of the Invention
The present invention relates to an ignition coil for an internal combustion engine and, more particularly, to a stick-type ignition coil to be fitted directly in the plug hole of an internal combustion engine.
2. Description of Related Art
As an ignition coil, a stick-type ignition coil is known. It has a rod-shaped central core disposed in a housing, and a primary coil and a secondary coil wound respectively on a primary spool and a secondary spool made of resin. Resin is filled in the housing of the ignition coil as an electric insulator. The insulator not only provides electric insulation among individual members in the housing but also fills clearances between wires of the coils thereby to restrict movement or breakage of the coils which may arise from engine vibrations. As the insulator, a thermosetting resin such as epoxy is used in consideration of the heat resistance. The ignition coil further has a permanent magnet attached to at least one of the two longitudinal ends of the central core to raise a voltage to be supplied to a spark ignition plug.
In this type of ignition coil, the central core contacts not only the resin insulator but also a case member such as a spool enclosing the outer circumference of the central core. The central core and the resin insulator or the case member, as having different thermal expansion coefficients, may repeatedly expand and contract as the surrounding temperature rises and falls. Then, the resin insulator or the case member, as contacting with the central core, especially the resin insulator or the case member contacting the longitudinal end corners of the central core, may crack, which results in defective electric insulation.
When the resin insulator or the case member around the central core cracks, an electric discharge may occur through the cracks between the secondary coil or a high voltage terminal (high voltage side) and the central core (low voltage side). If the discharge occurs between the high voltage side and the central core, the electric insulation between the high voltage side and the central core is broken to lower the voltage to be generated in the secondary coil, thus disabling a generation of desired high voltage.
If the central core and the resin insulator or the case member repeatedly expand and the contract due to changes in the temperature, the central core is caused to receive a load in the radial direction and in the longitudinal direction from the resin insulator and the case member due to the difference in the thermal expansion coefficient. Especially when the central core receives the load in the longitudinal direction, the magnetic permeability of the core may drop causing magneto-striction which disables generation of a required high voltage.
It is desired in a stick-type ignition coil to dispose an outer core around the outer periphery of the primary spool and the secondary spool. Since this outer core contacts directly with the insulator in the housing, the outer core and the insulator having different thermal expansion coefficients, may repeat expansions and contractions as the temperature changes. As a result, the insulator contacting with the outer core may crack causing an electric discharge between the secondary coil or a high voltage terminal the outer core. This discharge lowers the high voltage to be applied to the ignition plug.
In another ignition coil disclosed in Japanese Utility Model Publication No. 59-30501, although not a stick-type, the corners of the core are covered by over-coating the surface of the core with an elastomer. This prevents the corners of the core and the insulator made of epoxy resin from coming into direct contact with each other and suppresses the cracks in epoxy resin in the vicinity of the corners of the core. This over coating is not applicable to the stick-type ignition however because the stick-type is so regulated in its external diameter as to match the internal diameter of the plug hole.
It is an object of the present invention to provide an ignition coil capable of suppressing drawbacks caused by changes in surrounding temperature.
It is another object of the invention to provide an ignition coil capable of suppressing cracks from occurring in the vicinity of the longitudinal end corners of the a central core and/or outer core.
It is a further object of the invention to provide an ignition coil capable of suppressing dielectric breakdown caused by changes in surrounding temperature.
According to the first aspect of the invention, an ignition coil has an elastic buffer member at at least one of longitudinal end corners of a central core to absorb a difference in thermal expansion coefficients between the central core and a resin insulator or a case member such as a spool. As a result, even if the resin insulator or the case member having the thermal expansion coefficient different from that of the central core repeatedly expands and contracts together with the central core as the temperature changes, the resin insulator and the case member in the vicinity of the longitudinal end corners of the central core can be prevented from cracking. Alternatively, at least one of the two end corners of the central core may be surrounded by a space, so that a case member such as a spool or a resin insulator enclosing the outer circumference of the central core is not in contact with the longitudinal end corners of the central core.
According to the second aspect of the invention, an ignition coil has an angled member to cover the inner circumference corner of the longitudinal end of an outer core which is arranged around the outer circumferences of a primary coil and a secondary coil, so that a resin insulator is restricted from coming into direct contact with the inner circumference corner of the outer core. As a result, even if the outer core and the resin insulator, having different expansion coefficients, repeatedly expands and contracts as the temperature changes, cracks can be suppressed in the resin insulator in the vicinity of the inner circumference corner of the outer core. As a result, the electric discharge can be suppressed so that the drop in the voltage to be applied to an ignition plug can be restricted. Alternatively, the spool may have a flange to be arranged to cover the longitudinal end corner of the outer core, so that the cracks, if caused in the resin insulator in the vicinity of the inner circumference corner of the outer core, will hardly extend to the inner circumference because of being shielded by the outer spool. As a result, the cracks are less likely to reach electric wires connecting the coils and terminals in the ignition coil electrically.
According to the third aspect of the invention, an ignition coil has a separating member to separate a spool and a resin insulator from each other so that the spool and the resin insulator disposed inside and outside of the separating member can expand/contract separately from each other with a change in temperature. Thus, the spool and the resin insulator are prevented from cracking in a peripheral part on which a large force is liable to act.
According to the fourth aspect of the invention, a resin material used for at least an inner one of a primary spool and a secondary spool contains more than 5 weight % of rubber component. Accordingly, even if the inner spool is hindered from contracting toward the inside more than a coil wound thereon in low temperature by adhesion, it can reduce the distortion and can extend while maintaining the adhesion with the coil, thereby restricting the inner spool from cracking.
According to the fifth aspect of the invention, an ignition coil has an insulator made of a flexible material to hold individual members adhered to one another even if the members having different thermal expansion coefficients expand and contract as the temperature changes. Preferably, an average of the thermal expansion coefficient at xe2x88x9240xc2x0 C. to 130xc2x0 C. is set within a range of 10 to 30 ppm in a test method corresponding to ASTMD790, so that a thermal expansion coefficient of the insulator becomes close to that of iron or copper used for a core or coils, thus restricting distortion of spools and the insulator.